Gaskets having both polymer (PTFE) and metal components have been known and used for many years. These types of gaskets are acceptable for many gasket applications. Typically, a corrugated metal insert is combined with polymer layers, tapes, or the like to form a gasket able to seal with lower bolt loads, provide improved thermal cycling, and withstand increased pressure resistance.
Conventional prior art gaskets will often have polymer rings sandwiched around a corrugated metal insert. The rings are attached to the insert and/or to each other by some form of adhesive. This sandwich-type construction has historically meant that the gasket has seams at the inside diameter (ID) and/or outside diameter (OD) of the gasket. All of the layers of the gasket, including the metal and adhesive layers, are exposed at the inside diameter and outside diameter of the gasket. Problems with these types of gaskets include corrosion or degradation of the various components of the gasket that may occur as a result of the exposure of the metal and/or adhesive to the process temperature and media in the system in which the gasket is used. Many industries including semiconductor, food and beverage, pharmaceutical and specialty chemicals cannot tolerate the potential for process contamination that exists with the metal and/or the adhesive being in contact with their process media. In applications above the softening or melt point of an adhesive (˜200° F.) a typical failure mode or limitation is that the adhesive is corroded away, and the gasket seal may fail or require re-torquing as a result of the reduced bulk of the gasket. These higher temperatures may also result in blow-out failures when the adhesive is melted or softened.
If an insert is exposed, or is eventually exposed, to the environment, media, or other conditions in the system that the gasket is placed, prior art gasket construction requires the use of metal inserts that are chemically compatible with the process being sealed. For many corrosive chemical applications where the tightness, pressure resistance and resiliency of this type of gasket is required, exotic alloy inserts such as hastalloy, titanium, and other similar products are required. The resulting gaskets are very expensive and the required adhesive layers will still suffer from the same thermal degradation or chemical corrosion as described above. Both of these limitations may limit or prevent the use of the gasket design and the resultant performance benefits. Also, while exotic alloy inserts may allow the use of these gaskets in harsh chemical applications, there is no means of making this design suitable for use in applications like semiconductors, food and beverage, or specialty chemicals where contamination from the adhesive is the limiting factor.
FIG. 1 illustrates one prior art gasket that was developed to minimize the limitations of the earlier style corrugated insert gasket stated above. In this construction methodology, the gasket includes a single polymer ring that has a slit in it that extends around the outside diameter from the outside diameter toward the inside diameter. The slit does not extend all the way through the polymer ring to the inside diameter. A metal insert is positioned in the pocket that is formed by the slit of the polymer ring. The metal insert may or may not have an outside diameter essentially the same as the polymer ring. This gasket design effectively isolates the corrugated insert at the ID from the process media thus eliminating one of the limitations with the original gasket design. Whether or not the metal insert is directly exposed to the outside diameter of the gasket, the general conditions in a plant or a system where the gasket is used may still attack or thermally degrade the adhesive and/or the metal insert via the slit in the outside diameter of the gasket. Additionally, application for this improved gasket in PTFE lined piping systems can be negated because of static electricity discharge between the exposed metallic OD of this gasket construction to the bolts that secure the flange together. Also, because glue or some form of adhesive is still required with this design, the gasket's use within piping systems or vessels where any type of contamination is undesired is still restricted. The manufacturing technology and throughput with this pocket style gasket stated above is very expensive, very labor intensive, and stringent quality control measures must be employed to insure that the slit never extends all the way to the ID. If this were to happen and go unchecked, this gasket could fail catastrophically in chemical services that are not compatible with the insert metallurgy.
Accordingly, despite the advancements made with the ID protection envelope detailed above, there remains a need for a completely seamless metallic or corrugated metallic insert gasket that does not contain any seams at the ID or OD and a cost effective method to create such a gasket which does not rely upon careful slitting of the envelope material that is placed around the insert. A seamless corrugated/metallic insert gasket would ideally prohibit or lessen the ability of a corrosive agent to attack or degrade any structure of the gasket. A seamless corrugated/metallic insert gasket and a method for making the same, in accordance with the present invention, would effectively address one or more of the foregoing or other drawbacks associated with prior art gaskets.
Another prior art PTFE gasket is the “Task-Line” type gasket where a perforated stainless steel (SS) insert is encapsulated within a full density (hard) PTFE matrix. This gasket is made using molds whereby PTFE resin is charged into the mold, the insert is then added, and more PTFE resin is added on top of that. The PTFE resin and insert are compressed under extreme pressure at elevated temperatures above the PTFE melt point temperature. The PTFE resin therefore forms a hard, solid mass encapsulating the insert. The finished gasket is very hard (the PTFE is at full density, about 2.2 gm/cc), there are virtually no recovery/resiliency advantages with this design, and creep of the virgin PTFE remains very problematic.
Accordingly, there is a need for a finished gasket with any desired PTFE “skin” density between an expanded PTFE density (typically about 0.6 gm/cc) and the theoretical “full density” of PTFE (about 2.2 gm/cc). In the prior art Task-Line gasket above, the PTFE resin is melted above the PTFE sintering temperature and flows/compacts together around the insert. There is needed the beneficial physical properties of the lower density expanded, porous or microcellular PTFE. An approximate 0.6 gm/cc density PTFE “skin” or facings around the insert are desirable for flange surface adaptability (conformability) and low stress to seal, while higher “skin” densities are desirable when the flange surfaces do not require a highly compressible facing material or cut-through resistance at higher stresses. The use of various expanded, porous, or microcellular PTFE components allows the final gasket to have much improved creep resistance over the Task-Line (virgin, hard) PTFE gasket. Also, improved gasket resiliency/springback is a major performance advantage of any corrugated insert PTFE gasket with soft PTFE facings. The hard PTFE skin of the Task-Line gasket negates any benefit of a corrugated insert, and thus there are no known commercial Task-Line products with a corrugated insert.